Electronic control circuits



I June 15, 1948. w. J, HELD 2,443,347

ELECTRONIC CONTROL CIRCUITS Filed NOV. 19, 1942 3 Sheets-Sheet 1 Fig 1.

cow/mum run" cum/EMT (IV/I.) Y

2' 44; 0'0 ab lo'o PL/ITE VOLTIIGE xiv \ ISnventor WILL MM u. Flt-L0 (Ittorneg June 15, 1948. w, HELD 2,443,347

ELECTRONIC CONTROL CIRCUITS Filed Nov. 19, 1942 3 Shets-Sheet 2 3maentor W/L Ll/7M J F/fLD.

' attorney June 15, 1943. 4 w. J. FIELD 2,443,347

ELECTRONIC CONTROL CIRCUITS I Filed Nov. 19, 1942 s Sheets-Sheet s Imnentor WILL/17M J -F//.0

attorneg Patented June 15, 1948 UNITED STATES PATENT OFFICE 1c Claims. (or. 250-27) The present invention relates to electronic amplifier circuits, and particularly to amplifier circuits adapted for use in electrical control systems.

An object of the present invention is to provide an improved amplifier circuit for use in electrical control systems.

Another object of, the present invention is to provide an amplifier circuit which may be supplied with electrical energy from an alternating current supply, without the use of rectifiers or converters.

A further object of the present invention is to provide a method for determining the proper load impedance and anode-cathode voltage to be used. in amplifier circuits including an electrical discharge device which is supplied with energy directly from an alternating current source.

Another object of the present invention is to provide an improved circuit for. producing alternating electrical signals. variable in phase in accordance with the polarity of a unidirectional signal applied to the circuit.

A further object of my invention is to provide an amplifier circuit of. the type described, in which the controlling tulidirectional potential is applied to the input circuit, and the output circuit is supplied with electrical. energy from an alternating current source. Astill further object is to provide, in the output circuit of the discharge device in saidcircuit, a parallel resonant circuit, and signal output terminals coupled to the terminals of saidparallel resonant. circuit.

A still further object of the present invention is to provide a circuit for converting unidirectiona1 signal potentials into alternating signal potentials of a phase dependent upon the polarity of the unidirectional potential, in which means are provided to maintain the direct current impedances in a network substantially balanced, while the alternating current impedances in the network are unbalanced upon a change in the direct current signal potential.

Other objects and advantages of my. invention will become apparent from a consideration of the appended specification, claims, and drawings, in which Figure 1 is an electrical wiring diagram of an electrical control system including an amplifier, and embodying certain principles of my invention,

Figure 2 is a graphical illustration of the method of determining the proper load impedance and anode-cathode potential for use in the amplifier circuit of Figure 1,

Figures 3 and 4 are graphical illustrations of certain operating conditions of the circuit disclosed in Figure 1,

Figure 5 is an electrical wiring diagram of a modification of my invention, and

Figures 6, 7, 8, and 9 illustrate further modifications of my invention, which are especially adapted. to transform. unidirectional signal potentials into alternating signal potentials.

Figuresl, 2, 3, and 4 Referring to Figure 1, there is shown a system in which a. controller, generally indicated at In, operates a slider I along a slide wire resistance l2. The slider H and the resistance l2 taken together, form a control potentiometer l4.

The controller It may be any device responsive to a variable condition. which is indicative of the need. for operation of a load. device I3. For example, the controller |0 may be a temperature responsive device, such as a thermostat, and the load device l3 may be a heater or other temperature changing means.

The resistance I2 is. connected inan. electrical network is, of the Wheatstone bridge type. The network |5 also includes a resistance It. A slider H cooperates with resistance l6, and these two elements together form a. follow-up potentiometer l8. The network l5 has a pair of input terminals and 2!, and the sliders H and H serve as. the output terminals. The input terminals 211 and 2! are connected through conductors 23 and 24,,respectively, to the terminals of a secondary winding 22 on a transformer 25. The transformer 25 also includes a primary winding 26 and secondary windings 21. and 28.

The slider l'! is fixed on a shaft 30, which is rotated by a reversible motor generally indicated at 3|, acting through a gear train schematically shown at 32. A driving connection is also provided between the gear train 32v and the load device l3.

The motor 3| includes an armature 33 and field windings 34 and 35. The motor 3| is of the series type, and is so constructed that when field winding. 34- is energized, the motor rotates in one direction, while when the field windin 35 is energized, the motor operates in the opposite direc- .tion,

The field Winding 34 may be energized through circuit which may be traced from the right hand terminal of abattery 36., through a. conductor 31, a switch arm 38, a contact 3.9, a conductor 40, winding 34., a conductor 4|, armature 33, and groundv connections 42. and. 43 to the. left terminal of battery 3.6. The. winding 35 may be energized through a similar circuit which may be traced from the right hand terminal of battery 36, through a conductor 31, switch arm 45, a contact a conductor 47, field winding 35, conductor 4|, armature 33, and ground connections 42 and 43 to the left terminal of battery 35. It will be readily understood by those skilled in the art, that the battery 35 may be replaced by any other suitable source of electrical energy.

The switch arm 38 is a part. of a relay 5|), which energization of winding The switch arm is is part of a relay 52, and its position is controlled by the energization of a relay winding 53.

An amplifier circuit, generally indicated at 55, is provided to amplify any potentials existing between the output terminals of the bridge circuit I5. The amplifier circuit 55 includes a twin triode 55, which may be of the type commercially known as type 7N7. The twin triode 56 includes a first triode 51 having an anode 58, a control electrode 59, a cathode 60, and a heater filament 6!. The twin triode 56 also includes a second triode 02 having an anode 63, a control electrode 64, a, cathode 65, and a heater filament 60.

The triodes 51 and 62 have a common input circuit which may be traced from either control electrode 59 or control electrode 64, through a conductor 50 or a conductor H respectively, a resistance l2, and ground connections 13 and 14 to either cathode 60 or 65. serves as one output terminal of bridge circuit I5 is connected through a conductor 15 and a blocking-condenser 1'6 to the input circuit of amplifier 55. The slider H, which serves as the other outputterminal of bridge circuit i5 is connected to ground at 11.

The output circuit of triode 51 may be traced from the upper terminal of transformer secondary winding 21 through a conductor 80, anode 5t, cathode fil'L ground connections 14 and 8|, and resistances 82, 83, and 84 to the center tap on transformer secondary winding 2'1. The output circuit of triode 62 may be traced from the lower termi-' nal on secondary winding 21 through conductor The slider H, which I 85,anode 63, cathode 65, ground connections 14 and 8|, and resistances 82, 83, and 80 to the center tap on secondary winding 21.

The resistance 83 is provided with a movable tap 86. A condenser 8'! is connected between tap B6 and ground at Bl. The tap 85 is connected through a conductor 88 to a discriminator circuit generally indicated at 90, which'causes selective energization of the relay windings 5i and '53, depending upon the phase of an alternating potential existing between the tap 86 and ground.

The circuit 90, which functions both as a discriminator circuit and as an amplifier circuit, includes a twin triode 9!, which may also be of the type commercially known as type 7N7. The twin triode 9! includes a first triode 92, having an anode 93, a control electrode 94, a cathode 95, and

a heater filament 96. The twin triode 9| also ineludes a second triode 91, having an anode 98, a control electrode 99, a cathode I00, and a heater filament 10!.

The triodes 92 and 91 have a common input circuit which may be traced from either control electrode M or control electrode 99 through conductor 88, tap 86, the portion of resistance 83 to the left of tap 86, resistance 82 and condenser 81 in parallel with the latter two resistances, and

I09 connected in parallel therewith, anode 98, cathode I00, and ground connections Hi2 and I01 to the center tap on secondary winding 28.

The heater filaments E1, 56, S6, and i0! may be energized from any suitable source of electrical energy (not shown) The following table shows values of the various resistances and condensers in the circuit of Figure 1 which have been used in one embodiment of the invention:

Capacitance or 5,000 ohms. 5,000 ohms. 8,000 ohms.

l mogohm.

.05 microfarad. 3,000 ohms. 10,000 ohms.

- 10.000 ohms.

.01 microfarad. l microfarad.

Operation of Figures 1 to 4 Consider first the operation of the amplifier circuit 55. Since the anode to cathode potential of the triodes 51 and 62 is continuously varying, it should be apparent that this circuit is not well adapted to the use of conventional methods for determining the proper load impedance and plate voltage to be used. By the term load impedance, is meant the impedance in the output circuit of either triode externally of the triode itself. By the term plate voltage, is meant the anode to cathode voltage of the triode when no current is flowing therethrough. By the term plate current is meant the current flowing through the output circuit of the triode.

Because of the continuously varying anode to cathode voltage obtained in this circuit, I find it preferable to choose the values of load resistance and plate voltage so as to obtain a maximum change in the plate resistance of the tube per unit change in the voltage applied between the control electrode and the cathode. This is in contrast to the conventional method of constructing an amplifier circuit, in which the object is to obtain the highest possible amplification factor consistent with considerations of distortionless amplification.

My method of calculating the proper load impedance and plate voltage to produce the optimum operation of the amplifier circuit is illustrated in Figure 2. There is shown in Figure 2 a family of static characteristic curves for one half of a type 7N7 twin triode. As is well known in the art. each of these curves shows the variation of plate current obtained as the plate voltage is varied and the control electrode to cathode voltage (represented in Figure'2 by Be) is maintained at a fixed value. The slope of each of these curves atany point is a measure of the plate resistance of the triode under the particular conditions obtained at that point. In other words, the slope represents the diflerential change in plate current divided by the difierential change in plate voltage. I choose a point of substantially maximum slope, as indicated at A on the curve representing the static characteristic at the maximum expected value of control electrode potential, and a point of substantially minimum slope, as indicated at B, on the curve representing the static characteristic at the minimum expected value of control electrode potential. Through the two points" so selected, I draw a straight line, and extend this line until it intersects the plate voltage axis. The slope of this line is then a measure of the proper load impedance to be used, and the point at which this line intersects the plate voltage is a "measure of the proper peak value of anode to cathode voltage to be used.

Since only one of the triodes 5i and 62 may be conductive on any given halfoycle, the same load impedance may be used for both the triodes 51 and 62. This load impedance comprises the resistances 82, 83 and 84.

Figure 3 illustrates the conditions obtained when no signal is impressed on control electrodes '59 and 64. Referring to Figure 3, there are shown curves C representing the current flow taking place through the triode 5'! on alternate 'half cycles, and curves D showing the current flow taking place through the triode on the opposite half cycles. The curves C and D taken together represent the current flowing through the resistances 32, 83, and M. Because of the smoothing eflect of the condenser 81, the current flow through the resistance 82 and the portion of the resistance to the left of tap 36 takes place substantially as represented by the curve E in Figure 3. The curve E may be considered as comprising two components, a unidirectional component F of constant value, and the other an alternating component having twice the frequency of the energy supplied by winding Figure 4 shows the current and potential con" ditions existing in the output circuit of amplifier stage when a signal in phase with the anodecathode voltage of triode 5! is applied to the control electrodes 59 and 54. If the signal is in phase with the anode-cathode voltage or triode 57, the control electrode 59 is thereby made more positive during the half cycles when the triode 5'! may be conductive, and its conductivity is thereby-increased during those half cycles. The current flow through the triode 5! under such conditions is represented in Figure l by the curve G. On the other hand, the signal potential is negative during the half cycles when the triode '52 may be conductive, and hence the signal potential operates to reduce the conductivity of the triode E2. The current l'low through the triode 52 is represented in Figure e by the curve H. The current flow through the load impedance comprising the resistances til, and 82 and the condenser i3? is represented by the curves G and H taken together. The current flow through resistance 62 and the left portion of resistance 83, and hence the potential between tap '85 and ground represented in Figure 4 by the curve K, which differs from the curves G and H by reason of the smoothing action of condenser $37. The curve K may be considered as comprising a unidirectional component L of substantially constant value, and alternating component of t.e.sa1ne frequency as that of the energy supplied by w.-. "rig 2?. It should be iurtlren apparent that the phase or the alter hating component of the potent 1K is dependent upon the phase of the alternating signal in:" pressed on. the input circuit of amplifier 55.

It may be noted from t that the potcntial K is not pure sine wave, but that its form is considerably distorted. It has been found that the distortion obtained with this type of amplifier circuit is not suilicient to give trouble in a control system such as the one illustrated in Figure 1.

Referring now to the discriminator circuit 9t of Figure 1,:it may be seen that the output *potential existing between tap 86 and ground 'is impressed on the input circuit of the discriminator 96. The unidirectional potential existing between tap 86 and ground is of a polarity such that, as impressed on the input circuit of the discriminator Bil, it acts to bias the control electrodes '34 and 9.9 negatively with respect to their associated cathodes. This negative potential may varied by adjusting the tap along -re sistance It is not necessary that this potential be great enough to .maintain the triodes 52 and '9 continuously cut oil, but it should be sufilcientto maintain the normal output current of those triodes below the value necessary to cause effective energization of relay windings 51 and 53.

"When the alternating component of the signal potential between tap 86 and ground is in phase with the anode-cathode potential of triode 92, the conductivity of that triode is increased. If the signal potential is large enough, the current flow through the triode 92 becomes suflicient to energize relay winding 53. This current flow occurs only on alternate half cycles, but is held over by means of the condenser H16 .50 that the relay winding is substantially continuously energized. Similarly, if the signal potential is in phase with the anode-cathode potential of triode 91, the relay winding 5! is energized.

tarting with the sliders H and H in the positions shown in the drawing, let it be assumed that the slider l! is moved to the left along resistance l2 by the controller Ill. The potential of slider 35 therefore approaches that of bridge input terminal 25 while slider I! remains at ground potential. Therefore a signal potential exists between sliders H and ll which is of the same phase as would exist if the slider H were connected to the upper terminal of secondary winding 22 and slider I! were connected directly to the lower terminal. This signal potential therefore has its positive half cycle at the same time that the potential applied to the anodecathode circuit of triode 5! and is such as to make the triode 5? more conductive. The current flow in the output circuit of the amplifier stage 55 therefore takes place as indicated in Figure 4. It will be readily understood that the potential at tap 86 increases negatively as the current flow in the output circuit of amplifier 55 increases. Therefore the alternating component of the current flow as expressed by the curve K in Fig 4 causes a potential of opposite phase to appear on the control electrodes 94 and 99 of the discriminator stage 96. Then, under the conditions of Figure 4, during the half cycles when the triode 57 is conductive, the potential applied to control electrodes 94 and 93 is highly negative, and maintains the triodes 52 and 9'! non-conducting. During the following half cycle, during which the triode ill may be conductive, the potential of the control electrodes 94 and 99 becomes more positive, and the triode 9'! becomes more conductive, thereby energizing relay winding 5! Energization of relay winding 5! causes switch arm 38 to engage contact 39, thereby completing the energizing circuit for winding 34 and armature 33 of motor 3L Completion of this circuit causes operation of motor 3! in a direction to drive slider l! to the left along resistance [5, so as to reduce the potential difference between sliders l I and I1 and thereby robe-lance the bridge circuit l 5. As this difference of potential is decreased, thesignal applied to the amplifier 55 is decreased, and when the bridge I is rebalanced, the signal applied to amplifier 55 is no longer sufficient to maintain energization of relay winding 5|. The energizing circuit for motor 3| is interrupted by movement of switch arm 38 away from contact 39, resulting in the stopping of motor 3I and hence in the motion of slider I1.

In a similar manner, it may be understood that if the slider II is moved to the right along resistance I2 a signal of the opposite phase is impressed on the input circuit of amplifier 55, and causes energization of relay winding 53. This causes completion of the energizing circuit for motor 3| which includes field winding 35, thereby causing rotation of motor III in the opposite direction, and movement of slider I1 to the right so as to rebalance the bridge circuit I5.

Figure 5 r In Figure 5 I have shown a modification of the control system of Figure 1, in which the amplifier and discriminator circuits are somewhat different from those used in the system of Figure 1. In Figure 5, I have used the same reference numerals as in Figure 1 wherever the particular element is exactly equivalent to the corresponding element of Figure 1. These parts of the system of Figure 5 will not be described in detail.

The amplifier circuit in the system of Figure 5 is generally indicated at I29. Amplifier circuit 529 includes a twin triode IZI, which may be of the type commercially known as type 7N7. The twin triode I2I includes a pair of triodes I22 and I23. The triode I22 has an anode I24, a control electrode I25, a cathode I26, and a heater filament I21. The triode I23 has an anode I28, a control electrode I29, a cathode I39, and a heater filament I3I.

The control electrode I25 is connected to a conductor I35, and the controlelectrode I29 is connected to a conductor I36. The conductors I35 and I35 are connected through a variable resistance I31 and a fixed potentiometer resistance I38. A movable tap I39 associated with resistance I39 is connected through a conductor I 49 to slider I I oi'control potentiometer I4.

The input circuit of triode I22 may be traced from control electrode I25 through conductor I35, resistance I38, tap I39, conductor I49, slider,

II, network I 5, slider I1, and ground connections 11 and I4! to cathode I26. The input circuit of triode I23 may similarly be traced from control electrode I29 through conductor I36, resistance I38, tap I39, conductor I49, slider II, network I5, slider I1, and ground connections 11 and IM to cathode I39.

The output circuit of triode I22 may be traced from the upper terminal of secondary winding 21 through a resistance I43, a conductor I44, anode I24, cathode I25, ground connections MI and I45, and 'a resistance I46 to the center tap on transformer secondary winding 21. The output circuit Of triode I23 may similarly be traced from the lower terminal of secondary winding 21 through a resistance I41, a conductor I 48, anode I28, cathode I 39, ground connections I4! and I45, and resistance I46 to the center tap on winding 21. A condenser I49 is connected between the upper terminal of secondary winding 21 and ground to balance the distributed capacitance between the winding 21 and ground, which may be considered as a single lumped capacitance I59 connected between the lower terminal of winding 21 and ground.

The control electrode I25 is biased negatively with respect to its cathode I26 by a connection through a resistance I90, conductor I48, and resistance I41 to the lower terminal of secondary winding 21. The control electrode I29 is similarly biased negatively by a connection through a resistance I9I, conductor I44, and resistance I43 to the upper terminal of secondary winding 21. Since each control electrode is connected through the circuit just traced to the opposite end of the secondary winding from that to whichthe anode associated with that control electrode is connected, it may be seen that during the half cycles when the anode is positive with respect to the cathode, the control electrode is maintained at a negative potential with respect to the cathode by this biasing connection. The magnitude of this bias is controlled by the resistance I31. It may be noted that a circuit, which is in efiect a voltage divider circuit, may be traced from the upper terminal of transformer winding 21 through resistance I43, conductor I44, resistance I9I, conduc'tor I33, resistance I31 in parallel with resistance I38, conductor I35, resistance I99, conductor I48, and resistance I41 to the lower terminal of secondary winding 21. If the resistances I99 and I9! are equal and if the resistances I41 and I43 are equal, as would normally be the case, it may be seen that along this voltage divider connection just traced, the center point of the variable resistance I31 is at a potential corresponding to the potential of the midpoint of the transformer winding. By varying the resistance I31, the potentials of the control electrodes with respect to the center tap potential, and hence the potential of the control electrodes with respect to ground, may be adjusted. The resistance I31 therefore serves to adjust the biasing potential applied to the input circuit of amplifier I29, or in other words, it may be said to operate as a gain control for the amplifier.

The adjustable tap I39 is used to correct for any differences in the individual characteristics of the triodes I22 and I23. Th resistance I38 also functions as a protective resistance, inasmuch as it prevents the impedance between the control electrodes and ground from becoming equal to zero, as might happen if both sliders II and I1 reached corresponding ends of their resistances I2 and I6. If the control electrode I25 is connected directly to the cathode with no intervening impedance, it has been found that the controlling efiect of the control electrode becomes unstable and upon subsequent insertion of a positive potential into the control eelctrode to cathode connection, the response of the triode is unpredictable. Thiscondition of operation of an electric discharge device is sometimes referred to as the contact potential region of operation. Operation of the discharge device in this region may be avoided simply by inserting an impedance, such as the resistance I 38 between the control electrode and the cathode. This impedance should be permanently maintained in that connec'tion.

The discriminator circuit of Figure 5 is shown generally at I 59. The discriminator circuit I69 includes a pair of tetrodes I BI and I62, which may be gaseous electrical discharge devices of the type commercially known as type 2950. The tetrode IB'I includes an anode I63, a pair of control electrodes I64 and IE5, a cathode I56, and a heater filament I31. The tetrode I62 includes an anode I10, control electrodes HI and I12, cathode I13, and a. heater filament I14. The cathodes I65 and I13, and the control electrodes I34 and HI may all be connected to ground at I15. The input circuit of tetrode I65 may be traced from control electrode l6'5 through a'conductor H5. a resist- 'ance I11, a suitable source of biasing potential,

shown as a battery I18, and ground connections I19 and I15 to cathode W6. The input circuit of tetrode I62 may be traced from control electrode ing 53 and a condenser I84 connected in parallel therewith, anode IB3,-cathode I66, ground connections 15 and H35, and a switch I86 to the center tap on secondary winding 23. The output circuit of tetrode I82 may be traced from the lower terminal of secondary winding 28 through relay winding and acondenser 81 connected in parallel therewith, anode I10, cathode I13, ground connections I and I85, and switch I 86 to the center tap on. secondary winding 28.

The heater filaments I21, I31, I61, and I14, maybe energized from any suitable source of electrical energy (not shown) Since the tetrodes I6! and I62 are of the gaseous type, in Which-after initiation of adischarge between the cathode and anode, the control electrode is no longer effective as long as a voltage (positive at the anode) remains between anode and cathode. However, the anode voltage in this case is alternating and consequently will extinguish the ionization if the-grid circuits are properly biased. It is understood that those skilled in the art can rearrange the biasing scheme either by shifting the phase of the bias potential or by introducing a potential between cathode and shield grid. Curves of the operation with these latter biasing efiects are given in published data on the 2050 tube and need not be explained here.

The switch I83 is provided so that the output circuits of the tetrodes I 6! and I 62 may be opened during the time the system is being started up, so that the tetrodes I6! and IE2 will not be damaged by the application of potential between the anodes and cathodes before the filaments are heated.

The operation of the system of Figure 5 is'entirely analogous to that of the system of Figure l, and it is believed that further description of the operation of this system is unnecessary.

The following table shows values of the various circuit elements in the circuit of Figure 5 which have been used in one embodiment of that circuit:

Reference Numeral Quantity 1 megohm.

l-megohm.

approximately 10,000 ohms. 0 ohms.

approximately 10.000 ohms.

149... 600 to 1600 micro'iarads. 130 5megohms. l l

'5 megolnns.

.02 microfarad. .02 microfarad.

If no value isgiven above for any'element, the

value for the corresponding element in Figure 1 may be used.

Figure 6 There is shown in Figure 6 a circuit in which an amplifier of the type described in connection with Figure 1 is used to convert a unidirectional signal potential to an alternating signal potential, In the circuit of Figure 6, a somewhat different type of load impedance is used in the output circuit of the amplifier.

Referrin to Figure 6, there is shown an electrical network 200, of the Wheatstone bridge type, whichinclud'es a control potentiometer 20I and a follow up potentiometer 202. The control potentiomet'e'r20l includes a slider 203 and a slidewire resistance 204. The follow up potentiometer -202 includes a slider 205 and a slidewire resistance 208. The bridge circuit 200 has input terminals 201 and'2il3, which are connected through the left and 'right'terminals respectively of resistanc'es 204'and 206 to conductors 209 and 2| 0. The bridge circuit 200 is supplied with unidirectional electrical energy from any suitable source,

which is shown, by way of example, as a battery The sliders 203 and 205, which serve as the output terminals of the network 200 are conductively coupled to an amplifier circuit generally indicated at 2I5 through conductors 2I6 and 2I1, respectively. Amplifier circuit 2 I 5 includes a twin triode 2I8, which may be of the type commonly known as type 7N7. Thetwin'tri'ode 2 I'8-includes two single triodes 2I9 and 220. The triode 2| 9 has an anode 22I, a control electrode 222, a cathode 223, and a heater filament'22'4. The triode 220 has'an anode 225, a control electrode-226, a cathode 221, and a heater'filame'nt 228.

The input circuit of triode 2I9 may be traced from control electrode 222 through a conductor 230, a resistance 23I, a battery 232, to cathode 223. The input circuit of triode 220 may be traced from control electrode 226 through a conductor -233,'a resistance 234, and a battery 232, to cathode 221. The'battery 232 serves as a source of biasing potential for the triodes 2I9'and 220.

The output circuits of the triodes 2I9 and 220 are supplied with electrical energy from the secondary winding 235 of a transformer 236 having a primary winding 231, which may be connected to any convenient source of alternating electrical energy. The secondary windin 235 supplies current to a voltage divider circuit which may be traced from the upper terminal of winding 235 through a condenser 240, a resistance 24I, a. resistance 242,'and a condenser 243 to the lower terminal of secondary winding 235. The condensers 240 and 243 are blocking condensers, and serve to keep unidirectional electrical energy out of the transformer winding 235. where it might cause saturation of "the transformer core.

The output circuit of triode 2H2 may be traced from the upperterminal of resistance 2M through a conductor 244, anode 22!, cathode 223, a conductor 245, a parallel resonant network 255 including 'a condenser 245-and an inductance 241,

anda conductor '248t0 the common terminal 249 of resistances 24! and 242. The output circuit of triode 220 may be similarly traced from the lower terminal of resistance 242 through a'condoctor 250, anode 225, cathode 221, conductor 2 45, network 2-55,'and conductor 248, toterminal 249. The left and right terminals of the parallel resonant network are connected through blocking condensers 25I and 252 to alternating signal output terminals 253 and 254.

The following table shows values of the various ii circuit elements in Figure 6 which have been used in one embodiment of thatcircuit:

Table for Figure 6 Reference Numeral Quantity 1000 ohms. 1000 ohms.

1 megohm.

l Inegolim.

1 microfarad. 5000 ohms. 5000 ohms.

1 microiarad.

1 microfarad.

7 henries.

0.1 microfarad. 0.1 microi'arad.

Operation of Figure 6 is balanced, the current flow conditions in the output circuits of triodes H9 and 220 may be regarded as represented by the curves of Figure 3. Under these conditions, let it be assumed that the slider 203 moves to the right along resistance 204, thereby unbalancing the bridge circuit 200, and creating a difference of potential between sliders 203 and 205 of a polarity such that slider 203 is positive with respect to slider 205, as indicated by the legend applied to the input terminals 201 and 208 of the bridge circuit 200. This potential difierence between sliders 203 and 205 causes a current flow through resistances 23! and 234 in a sense such that their upper terminals are positive with respect to their lower terminals. The potential drop across resistance 23! opposes the biasing potential of the battery 232 in the input circuit of triode H9, and renders the triode more conductive. On the other hand, the potential drop across resistance 234 aids the biasing potential of battery 232 in the input circuit of triode 220, and therefore decreases the conductivity of that triode. The current flow in the common portion of the output circuits of triodes H9 and 220 now has an alternating component similar to that indicated by the curve K in Figure 4. The parallel resonant network 255 is tuned to the frequency supplied by transformer winding 235. Therefore this network presents a high impedance to the alternating component of current flow in the output circuit which is of the same frequency. A correspondingly high potential drop of that frequency is therefore developed across the terminals of the resonant network 255 by this current. This potential is transmitted through the blocking condensers 25i and 252 to the signal potential output terminals 253 and 254. The output terminals 253 and 254 may be connected to a discriminator circuit which may, for example, be one of those illustrated in Figures 1 and 5.

If the bridge circuit 200 becomes unbalanced in the opposite sense, by movement of slider 203 to the left from the position shown in the drawing, the current flow through resistances 231 and 234 is of the opposite polarity and increases the conductivity of triode 220 while decreasing the conductivity of triode 2l9. The alternating po-' tential developed across the parallel resonant network is therefore of the opposite phase'from that previously obtained when the network '200 was unbalanced in the opposite direction.

Figure 7 In Figure 7 is illustrated a modification of the circuit of Figure 6. In this modification, the triode 220 has its control electrode 220 connected directly to the cathode 221 through a resistance 260, so that the triode 220 is always equally conductive. The slider 205 is connected to ground at 25! through a resistance 253, and the cathodes 223 and 221 are connected to ground at 232. The resistances 260 and 263 are provided to prevent unstable operation of the triodes, and function in a manner similar to resistance I38 of Figure 5. Any unbalance potential existing between the sliders 203 and 205 is applied between the control electrode 222 and cathode 223 of triode 2i9. When the bridge circuit is balanced, the output circuits of both triodes are equally conductive, since their respective control electrodes are at cathode potential. The current flow through the parallel resonant network 255 is therefore substantially unidirectional, with a small double frequency'component. Since the network 255 has a very low impedance to the double frequency component, the double frequency voltage appearing across terminals 253 and 254 is very small.

If the bridge circuit becomes unbalanced, the conductivity of triode 2i9 is either increased or decreased from its normal value, thereby creating an alternating component in the current flowing in the output circuit. This alternating component has the same frequency as the supply, to which frequency the resonant network 255 is tuned.

When the conductivity of triode H9 is increased abcve its normal value, the current fiow through the triode 2l9 represents the positive half cycle of current flow, while the current flow through triode 220 represents the negative half cycle of current flow through the network 255. On the other hand, if the conductivity of triode 2!!) is decreased, the current supplied by that triode to the networkv 255 is smaller than that supplied by the triode 220, and hence the smaller current represents thenegative .half cycle and the current supplied by triode 220 represents the positive half cycle. It may therefore .be seen that this circuit produces an alternating signal potential at the output terminals 253 and 254 whose phase depends on the direction of unbalance of the bridge circuit.

Figure 8 In Figure 8, is shown a modification of the circuit of Figure 7, in which the triode 220 has been replaced by a constant impedance 270. The resistance 270 should preferably be designed to match the internal impedance of the triode 2 i 9. Furthermore, the parallel resonant network 255 has been replaced by a source of unidirectional potential, shown as a battery 27 l The battery 2' is .provided to stabilize the unidirectional potential existing between the output terminals 253 and 254. It has been found, that upon large unbalances of a bridge circuit of the type shown at 200, or more generally speaking, upon the application of large unidirectional potentials to the input circuit of the triode 2i9, the unidirectional potential existing across the network 255 changes sufficiently so that the change is reflected through the blocking condenser to the discriminator circuit and may cause erratic operation of that circuit; The

13 provision of a battery 2', or other source of a constant unidirectional potential in the output circuit of the amplifier tends to stabilize this unidirectional potential, and prevent such erratic operations of the system.

Figure 9 In Figure 9 is shown another amplifier circuit of the same general type as those disclosed in the previous figures. In the circuit of Figure 9, additional steps have been taken to maintain the unidirectional potential condition in the output circuit of the triodes 2 l9 and 22B stable while the alternating potential conditions vary in accordance with the unbalance of the bridge circuit 200.

Referring to Figure 9, the output circuit of triode 219 may be traced from the upper terminal of secondary winding 235 through a conductor 2'80, anode EZI, cathode 223, a portion of a resistance 28!, a tap 282, ground connections 283 and 284-, a battery 285, and a resistance 286 to the center tap of secondary winding 235. The output circuit of triode 220 may similarly be traced from the lower terminal of secondary winding 235 through a conductor 28?, anode 225, cathode i221, resistance 28%, tap 282, ground connections 283 and 284-, battery 285, and resistance 2-86 to the center tap on transformer winding 235.

It has been found that the provision of the resistance 226 in series with the battery 285 in the common portion of the output circuits of the triodes H9 and 2213 adds to the stability of the D. 0. condition in the circuit. The resistance 28? also functions to that end. Furthen more, differences between the characteristics of the two halves of the twin triode 218 may be corrected by moving the tap 282 along resistance 28!.

Since the operation of the circuit of Figure 9 is very similar to the operation of the circuits previously discussed, and it is believed that further description is unnecessary.

While I have shown and described certain preferred embodiments of my invention, other mcdificati'ons thereof will be readily apparent to those skilled in the art, and I therefore wish my invention to be limited only by the appended claims.

I claim as my invention:

1. Electrical amplifier circuit means, comprising in combination, at least two amplifier stages connected in cascade, the first of said stages comprising a pair of electrical discharge devices, each having an anode, a cathode, and a control electrode, an input circuit for each said device including its control electrode and cathode, an output circuit for each said device including its anode and cathode, a source of alternating electrical energy, means connecting said output circuits in phase opposition to said source, common load impedance means connected in both said output circuits, means for applying an alternating electrlcal signal potential to said input circuits, said first stage being efiective to produce across said common load impedance means a potential having' a unidirectional component and an alternating component corresponding in magnitude to said signal potential, the second of said stages comprising an input circuit and an output circuit, and means for impressing the potential across said common load impedance means on said second stage input circuit so that said unidirectional component serves as a biasing potential 101- said second stage.

2. Electrical control apparatus, comprising in combination, a control device, means responsive to a predetermined controlling condition for producing upon a change in said condition an electrical signal potential having an instantaneous polarity corresponding to the sense of change of said condition, amplifier circuit means and discriminator circuit means, each said circuit means including a pair of electrical discharge devices, each discharge device having an anode, a cathode, and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, power supply means including a transformer having a pair of secondary windings, means connecting one of said windings to supply alternating electrical energy to the output circuits of said amplifier circuit means in phase opposition, common load impedance means connected in both the output circuits of said amplifier circuit means, the value of said load impedance means and the potential of said one transformer winding being chosen so as to provide a maximum change in the plate resistance of said amplifier discharge devices per unit change in the potentials of their control electrodes, means connecting the other of said windings to supply alternating electrical energy to the output circuits of said discriminator circuit means in phase opposition, means coupling said signal potential producing means to the input circuits of said amplifier circuit means, means coupling said load impedance means to the input circuits of said discriminator circuit means, and means connected in the output circuits of said discriminator circuit means and responsive to the difierence between the currents flowing in said discriminator output circults to control the manner of energization of said control device.

3. Electrical control apparatus, comprising in combination, a control device, means responsive to a predetermined controlling condition for producing upon a change in said condition an electrical signal potential having an instantaneous polarity corresponding to the sense of change of said condition, amplifier circuit means and discriminator circuit means, each said circuit means including a pair of electrical discharge devices, each discharge device having an anode, a cathode, and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, power supply means including a transformer having a pair of secondary windings, means connecting one of said windings to supply alternating electrical energy to the output circuits of said amplifier circuit means in phase opposition, load impedance means connected in the output circuits of said amplifier circuit means, means connecting the other of said windings to supply alternating electrical energy to the output circuits of said discriminator circuit means in phase opposition, means coupling said signal potential producing means to the input circuits of said amplifier circuit means, means coupling said load impedance means to the input circuits of said discriminator circuit means, and means connected in the output circuits of said discriminator circuit means and responsive to the difierence between the currents flowing in said circuits to control the manner of energization of said control device,

4. Electrical amplifier circuit means, comprismg in combination, a pair of electrical dischar e devices, each having an anode, a cathode, and a control electrode, impedance means, an input circuit for each device including its control electrode and cathode, an output circuit for each device including its anode and cathode and said impedance means, power supply means including a transformer secondary winding having terminals and a center tap, connections between each of said anodes and one of said terminals, 2. connection between both of said cathodes and said center tap, said power supply means and said connections serving to supply the output circuits of said devices with alternating electrical energy in phase opposition, connections between each of said control electrodes and the terminal associated with the opposite anode for biasing both of said control electrodes negatively, means for applying a signal potential to said input circuits, and electrical load means responsive to the current flow through said impedance means.

5. Electrical amplifier circuit means, comprising in combination, a pair of electrical discharge devices, each having an anode, a cathode, and a control electrode, impedance means, an input circuit for each device including its control electrode and cathode, an output circuit for each device including its anode and cathode and said impedance means, power supply means including a transformer secondary winding having terminals and a center tap, connections between each of said anodes and one of said terminals, a connection between both of said cathodes and said center tap, said power supply means and said connections serving to supply the output circuits of said devices with alternating electrical energy in phase opposition, connections between each of said control electrodes and the terminal associated with the opposite anode for biasing both of said control electrodes negatively, means for applying a signal potential to said input circuits, gain control means comprising a. variable impedance connected between said control electrodes, and electrical load means responsive to the current flow through said impedance means.

6. Electrical amplifier circuit means, comprising in combination, a pair of electrical discharge devices, each having an anode, a cathode, and a control electrode, impedance means, an input circuit for each device including its control electrode and cathode, an output circuit for each device including its anode and cathode and said impedance means, power supply means including a transformer secondary winding having terminals and a center tap, connections between each of said anodes-and one Of said terminals, a connection between both of said cathodes and said center tap, said power supply means and said connections serving to supply the output circuits of said devices with alternating-electrical energy in phase opposition, connections between each of said control electrodes and the terminal associated with the opposite anode for biasing both of said control electrodes negatively, an impedance connected between said control electrodes, a tap movable along said impedance, a source of signal potential connected between said tap and said cathodes, and electrical load means responsive to the current flow through said impedance means.

7. Electrical control apparatus, comprising in combination, a control device, means responsive to a predetermined controlling condition for producing upon a change in said condition an electrical signal potential having an instantaneous polarity corresponding to the sense of change of said condition, amplifier circuit means including a pair of electrical discharge devices, discriminator circuit means including at least one electrical discharge device, each discharge device having an anode, a cathode, and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, power supply means including a transformer having a pair of secondary windings, means connecting one of said windings to supply alternating electrical energy to the output circuits of said amplifier circuit means in phase opposition, load impedance means connected in the output circuits of said amplifier circuit means, means connecting the other of said windings to supply alternating electrical energy to said discriminator circuit means, means coupling said signal potential producing means to the input circuits of said amplifier circuit means, means coupling said load impedance means to said discriminator circuit means, and means connected in said discriminator circuit means and responsive to the time phase of the current flowing in said load impedance means to control the manner of energization of said control device.

8. An electrical circuit for producing alternating electrical signals variable in phase in accordance with the polarity of unidirectional signals applied thereto, comprising in combination, transformer means having a secondary winding provided with a pair of terminals and a center tap, a pair of impedance devices, at least one of said impedance devices comprising an electrical discharge device having an anode, a. cathode, and a control electrode, a connection between said anode and one of said transformer terminals, a connection between said cathode andthe other of said transformer terminals including the other of said impedance devices, load impedance means, a connection between said cathode and said center tap including said load impedance means, signal output terminals coupled to said load impedance means, and means to vary the conductivity of said discharge device relative to that of the other Of said impedance devices, said last named means comprising means for applying said unidirectional signals to said control electrode.

9. An electrical circuit for producing alternating electrical signals variable in phase in accordance with the polarity but not the magnitude of unidirectional signals applied thereto, comprising in combination, an electrical discharge device having an anode, a cathode, and a control electrode, a source of alternating electrical energy, a source of unidirectional electrical energy, a source of unidirectional signal voltage reversible in polarity, an input circuit for said discharge device including said control electrode and said cathode, an output circuit for said discharge device including said anode, said cathode, and said sources, means for impressing a unidirectional signal voltage from said source thereof upon said input circuit, signal output terminals, and means coupling said signal output terminals to said output circuit.

10. An electrical circuit for producing alternating electrical signals variable in phase in accordance with the polarity but not the magnitude of unidirectional signals applied thereto, comprising in combination, an electrical discharge device having an anode, a cathode, and a input circuit for said discharge device including said control electrode and said cathode, an output circuit for said discharge device including said anode, said cathode, said-sources and said impedance means inseries, means for impressing a unidirectional signal voltage from said source upon said input circuit, signal output terminals, and means coupling said signal ou put terminals to said output circuit.

11. An electrical circuit for producing alternating electrical signals variable" in phase in accordance with the polarity but not the magnitude of unidirectional signals applied thereto, comprising in combination; an electrical dis Charge device having an anode, a cathode; and a control electrode, a source of alternating electrical energy, a source of unidirectional electri cal' energy, asource of unidirectional signal voltage reversible in polarity, an input circuit'for said discharge device'including said controlielectrodeand said cathodaan'output circuit for said discharge device including said anode, said cathode, and said sources, impedance means common to said input and output circuits, means for impressing a unidirectional signal voltage from said source upon said input circuit,signal outputfter minals, and means coupling said"signal output terminals to said output circuit.

12. An electrical circuit for producing alternating electrical signals variable in phase in accordance with the polarity of unidirectional signals applied thereto, comprising in combination, transformer means having a secondary winding provided with a pair of terminals and a center tap, a pair of electrical discharge devices, each said device having an anode, a cathode, and a control electrode, connections between each of said anodes and one of said transformer terminals, load impedance means, a source of unidirectional electrical energy, a connection between both of said cathodes and said center tap including said load impedance means and said source, signal output terminals coupled to said load impedance means, a source of unidirectional signal voltage reversible in polarity, and means for applying said reversible signal voltage to said control electrodes to oppositely vary the conductivity of said discharge devices.

13. Electrical amplifier circuit means, comprising in combination, a pair of electrical discharge devices, each having an anode, a cathode, and a control electrode, impedance means, an input circuit for each device including its control electrode and cathode, an output circuit for each device including its anode and cathode and said impedance means, power supply means including a transformer secondary winding having terminals and a center tap, connections between each of said anodes and one of said terminals, a connection between both of said cathodes and said center tap, said power supply means and said connec tions serving to supply the output circuits of said devices with alternating electrical energy in phase opposition, connections between each of said control electrodes and the terminal associated with the opposite anode for biasing both of said control electrodes negatively, means for applying a signal potential to said input circuits, an impedance connected between said control electrodes, a source of signal potential connected between an intermediate point on said impedance and said cathodes, and electricalload means responsive to the current flow through 'sa'iddmpedan'celmeans; 14. Electrical control apparatus, comprising'in combination, a control device,m'eans responsive to a predeterminedcontrolling condition for producing upon a change in said condition an electrical signal potential having an instantaneous polarity corresponding-to the sense of'change of said'condition, amplifier circuit means anddiscr'iminator circuit means, eachsaid circuit means including a pair. of electrical discharge devices, each discharge device having an anode, a cathode, and a control electrode, a substantially unbiasedinput circuit for each discharge device including its" control electrode and cathode; an output circuit for each discharge device including its'an'ode and cathode, power supply means including 'a transformerhav ing a pair of secondary windings, means connecting one, of said'windings to supply alternating electrical energy to the-output circuits of said amplifiencircuit means in 'phase opposition, said output ci'r'cuitshaving aj common portion including a load impedance, means connecting the other of saidwindings to supply alternating electrical energy to tliefoutput circuits of; said discriminator circuit means in phase opposition, means'coupli'ngrsaid signal potential producing means to the input circuits of said amplifier circuit means, means. couplingan'adjustable portion of said load impedance to the'input circuits of said'discriininator circuit means to apply tosai'd input circuits a biasing voltage dependent in magnitude upon the magnitude of said adjustable portion and a further voltage dependent upon the condition of said signal potential producing means, and means connected in the output circuits of said discriminator circuit means and responsive to the difference between the currents flowing in said circuits to control the manner of energization of said control device.

15. Electrical control apparatus, comprising in combination, a control device, means responsive to a predetermined controlling condition for producing upon a change in said condition an electrical signal potential having an instantaneous polarity corresponding to the sense of change of said condition, amplifier circuit means and discriminator circuit means, each said circuit means including a pair of electrical discharge devices, each discharge device having an anode, a cathode, and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, power supply means including a transformer having a pair of secondary windings, means connecting one of said windings to supply alternating electrical energy to the output circuits of said amplifier circuit means in phase opposition, means connecting the control electrode of each discharge device of said amplifier circuit to the terminal of said winding to which the anode of the other discharge device is connected so as to bias said control electrode negatively during the conductive half cycles of the discharge device, load impedance means connected in the output circuits of said amplifier circuit means, means connecting the other of said windings to supply alternating electrical energy to the output circuits of said discriminator circuit means in phase opposition, means coupling said signal potential producing means to the input circuits of said amplifier circuit means, means coupling said load impedance means to the input circuits of said discriminator circuit means, and means connected in the output circuits of said discriminator circuit means andr'esponsive to the difierrence between the currents flowing insaid circuits to control the manner of energization of said control device.

16. Electrical control apparatus, comprising in combination, a control device, means responsive to a predetermined controlling condition for producing upon a change in said condition an electrical signal potential having an instantaneous polarity corresponding to the sense of change of said condition, amplifier circuit means and discriminator circuits means, each said circuit means including a pair of electrical discharge devices, each discharge device having an anode, a cathode, and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, power supply meansincluding a transformer having a pair of secondary windings, means connecting one of said windings to supply alternating electricalenergy to the output circuits of said amplifier circuit means in phase opposition, said output circuits having a common portion including a load impedance, means connecting the other of said windings to supply alternating electrical energy to the output circuits of said discriminator circuit means in phase opposition, means coupling said signal potential producing means to the input circuits of said amplifier circuit means,

a constant source of direct potential connected to said load impedance, means coupling said load impedance and said source of direct potential to the input circuits of said discriminator circuit means, and means connected in the output circuits of said discriminator circuit means and responsive to the difference between the currents flowing in said circuits to control the manner ofenergization of said control device.

WILLIAM J. FIELD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Theory and Applications of Electron Tubes, Reich, second edition, 1944, pages 263 to 265. (Copy in Div. 51.). 

